1. Field of the Invention
The present invention relates to a multimode radio communication system and, more specifically, to a multimode radio communication system capable of both indirect terminal-to-terminal communication performed through a base station and direct terminal-to-terminal communication performed not through a base station, for example, peer-to-peer communication.
2. Description of the Related Art
An example of a radio communication system is a pager system wherein a base station having a transmitting antenna of 100 meters or more in height is provided within every service area whose radius is about 30 kilometers. To call a specified terminal (pager) holder in this system, a caller dials the calling number of a pager of the terminal holder by telephone. In response to the calling number, a service center of a pager service company makes all the base stations send forth the calling number at once (or page the calling number). Upon receiving the calling number, the pager gives a calling sound to inform the terminal holder of the call. Recently a pager system capable of transmitting a message as well as a calling number, has been developed, in which not only is a calling sound produced but a message is displayed on a display of a terminal.
In either case, however, since the prior art pager system is directed to one-way communication from a base station to a terminal, another communication system, such as telephone, has to be employed when a called party wishes to respond to a received calling number or a received message. To eliminate this problem, a two-way pager system (or bidirectional pager system) capable of transmitting a response message from a terminal (pager) to a base station, has recently been developed.
According to one proposal of the two-way pager system, the same communication system as that of the normal one-way pager system is used in the down link from the base station to the pager, while a communication system different from that of the one-way pager system is adopted in the up link from the pager to the base station, as described in U.S. Pat. No. 5,335,246. The two-way radio communication between two pagers is achieved as follows: A transmission message is transmitted from one of the pagers to a service center of a pager service company using an up link through a base station. The service center makes all base stations transmit the transmission message at once (or page) through the down links. Upon receiving the transmission message, the other pager transmits a response message to the base station through the up link and then to the service center, as described above. The service center returns the response message to the former pager via the base station using the down link.
Moreover, there are a portable telephone system and a mobile telephone system as other radio communication systems for achieving two-way communication.
The foregoing communication systems are all based on the premise of the use of service provider infrastructure equipment such as base stations and telephone lines. Therefore, even when two terminals for two-way communication are close to each other and no infrastructure equipment is required, a signal always has to be transmitted via both a base station and a service center, with the result that the time for communication between the two terminals is lengthened and the infrastructure equipment runs to waste.